(a) Field of the Invention
The present invention relates to a compact vari-focal photographic lens system and, more particularly, to a compact vari-focal photographic lens system comprising a detachably attached auxiliary lens group which is to be used with popular cameras of which the lenses are not interchangeable and which need to be convenient for carrying.
(b) Description of the Prior Art
Recently, there is an increasing demand for popular cameras of which the lenses are not interchangeable, which are small in size and light in weight and which enable to take photographs easily and readily. Besides, as additional functions for popular cameras, various mechanisms are provided such as an automatic focusing mechanism, automatic film winding mechanism, date inserting mechanism (a mechanism for photographing the date together with the object), etc., and the utility value of popular cameras of which the lenses are not interchangeable is becoming still higher. However, as the popular cameras of which the lenses are not interchangeable need to be small in size, the lens systems which are to be used with this type of cameras and which are commercially available are mostly arranged as the fixed focal length type lens systems of which the focal length cannot be varied, and there exists almost no vari-focal lens system.
On the other hand, the rear converter type, inner converter type, etc. are known as the types of vari-focal lens systems.
Of said types, the rear converter type vari-focal lens systems are known, for example, as disclosed in Japanese published unexamined patent application No. 46224/82. However, in case of the rear converter type lens systems, the distance from the master lens to the image surface varies when the converter lens is inserted and, therefore, this type of lens systems are not suitable for such cameras as the popular cameras which are so constructed that it is difficult to move the stop and shutter.
The inner converter type vari-focal lens systems are known, for example, as disclosed in Japanese published unexamined patent application No. 108510/83. In case of the inner converter type lens systems, the converter lens is to be inserted in a narrow space in an intermediate portion of the lens system and, consequently, it is necessary to move a part of the master lens before inserting the converter lens. Moreover, to insert the converter lens to the intermediate portion of the lens system, a hole for inserting the converter lens should be provided at an intermediate portion of the lens mount and, consequently, the strength of the lens mount becomes insufficient.
Besides, the zoom type vari-focal lens systems are also known, for example, as disclosed in Japanese published unexamined patent application No. 128911/81. However, the zoom type vari-focal lens system are not suitable for the use with the popular cameras of which the lenses are not interchangeable.